As semiconductor device patterns have been increasingly miniaturized in recent years, an apparatus for measuring the dimensions of the device patterns with high accuracy, such as a critical dimension measurement SEM, has been requested to achieve a resolution of about 3 to 1 nm by using an electron beam with an acceleration voltage of 1 kV or less for the prevention of specimen destruction. To satisfy the request, it is necessary to converge the electron beam into a spot with a diameter not more than a desired resolution on a specimen surface. With such a low acceleration voltage, chromatic aberration in an objective lens presents a first problem. To reduce the chromatic aberration, various attempts have already been made by sophisticating the design of the objective lens.
In the 1990s, a retarding technology was introduced which increases an acceleration voltage for electrons by the magnitude of Vr to allow the passage of the electrons through the objective lens with a higher energy, while decelerating the electrons with the application of a voltage −Vr to a specimen to keep the energy of an electron beam low when it is incident on the specimen. The technology allows a reduction in chromatic aberration at the rate shown in Expression (1) where V0 is an electron acceleration voltage at an electron gun portion.
                              [                      Numerical            ⁢                                                  ⁢            Expression            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                            V            0                                              V              0                        +                          V              r                                                          (        1        )            
If Vr is excessively large, however, the specimen is destroyed by an electric field so that the magnitude of Vr which can be applied is limited. The chromatic aberration is also reduced accordingly if an electron source which emits an energy with a small width is used. At present, a field emission electron gun (with an emitted-electron energy width of about 0.3 eV), a Schottky field emission electron gun (with about 0.6 eV), and the like are used and an electron source having a smaller emitted-electron energy width and excellent stability is still in the search stage.
Since such a new electron source has not been commercialized yet, the reduction of the chromatic aberration in accordance with the foregoing methods is currently approaching the limit.
Of various approaches made to eliminate the limit, two have drawn attention, which are the use of a monochrometer and the reduction of the chromatic aberration using an aberration corrector. In relation to the approach using the aberration corrector, a technology which allows the compensation of the aberration in an objective lens by composing an aberration corrector from a combination of multipole lenses was proposed by Scherzer in the year 1947. In the 1970s, a specific structure had already been disclosed in Non-Patent Document 1 or Non-Patent Document 2. Recently, the technology has been experimentally verified by using a quadrupole/octupole aberration corrector of electromagnetic field type (Non-Patent Document 3).
An outline of the operation of this type of aberration corrector will be described with reference to FIG. 4. An aberration corrector 10 is composed of a multipole lens 11, an electromagnetic multipole lens 12, an electromagnetic multipole lens 13, and a multipole lens 14 to generate quadrupole fields and octupole fields in superimposed relation. In the aberration corrector 10, the quadrupole fields cause a converging effect and a diverging effect in respective two directions (the x-axis and the y-axis) perpendicular to an optical axis (the z-axis), which separate paraxial trajectories. In FIG. 4, the trajectories of electron beams are schematically shown by the fine lines. The first-stage multipole lens 11 causes the electron beams emitted from a crossover 41 to have trajectories in the x-direction (the x-trajectories each having one arrow in the drawing) diverged and trajectories in the y-direction (the y-trajectories each having two arrows in the drawing) converged, which are separated from each other. A trajectory in an arbitrary direction can be considered as a linear combination of these x- and y-trajectories. The second-stage electromagnetic multipole lens 12 is capable of generating a quadrupole electric field and a quadrupole magnetic field that has been 45° rotated from the quadrupole electric field relative to the optical axis in an x-y plane and compositely applying the electric field and the magnetic field in the x-y plane. The first-stage multiple lens 11 is excited such that the y-trajectories cross in the vicinity of the center of the electromagnetic multipole lens 12. At this time, the x-trajectories are apart from each other at a maximum distance to form a line image 42 extending in the x-direction at the center of the electromagnetic multipole lens 12. The excitation of the quadrupole of the electromagnetic multipole lens 12 has been adjusted such that the x-trajectories cross in the vicinity of the center of the third-stage electromagnetic multipole lens 13. At this time, a line image 43 presents a linear configuration extending in the y-direction. The x-trajectories and the y-trajectories separated past the fourth multipole lens 14 join at a crossover 44. The aberration corrector 10 is operated such that the crossover 41 is stigmatically formed into an image at the crossover 44.
At this time, the electromagnetic multipole lens 12 can be excited in the aberration corrector 10 by varying the ratio among the respective intensities of the quadruple fields including the electric and magnetic fields under the constraint that a resulting force exerted on an incident electron with an energy serving as a reference is not changed. In this case, an electron with an energy shifted from the reference is different in speed from the electron with the reference energy so that the exerted force changes if the ratio among the respective intensities of the electric and magnetic fields changes and the trajectories are displaced. The displacement is large in the x-direction apart at a distance from the optical axis and the trajectories are hardly affected in the y-direction extending toward the centers of the multipole fields. When the electron passes through the electromagnetic multipole lens 13, the x- and y-directions in the foregoing relation are reversed. In other words, only electrons with incidence energies shifted independently in the x- and y-directions can have trajectories changed by changing the ratio among the respective intensities of the quadrupole fields including the electric and magnetic fields in the electromagnetic multipole lenses 12 and 13.
By using this, the respective trajectories of an electron with a higher energy and an electron with a lower energy are preliminarily shifted outwardly and inwardly, each by an amount which allows the compensation of the chromatic aberration in the subsequently disposed objective lens, whereby the compensation of the chromatic aberration is performed. By generating octupole fields by using the multipole lenses 11 to 14 in addition to the quadrupole fields, it is also possible to compensate for the spherical aberration as disclosed in the articles 3 and 7 of Non-Patent Document 3.
In these well-known examples, an embodiment which uses a dodecupole for superimposing the quadrupole fields and the octupole fields is shown. In these well-known examples, an electro-optical system is constructed with a view to reducing even high-order aberration occurring inherently in the aberration corrector. However, since extremely small probes should be formed, total axial alignment in an entire electron beam apparatus is difficult when the aberration corrector is incorporated into the apparatus.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2000-195453
[Non-Patent Document 1] Optik 33 (1971), pages 1-24
[Non-Patent Document 2] Optik 83 (1989), pages 30 to 40
[Non-Patent Document 3] Nuclear Instruments and Methods in Physics Research, A 363 (1995), pages 316 to 325